1. Field of the Invention
The present invention relates in general to magnet designs. More particularly, the present invention relates to a magnet structure providing a Chiron Twister that produces a magnetic field capable of causing an electron to have a helical trajectory.
2. Description of the Prior Art
Magnetic fields are commonly used in the operation of numerous electronic devices, including those which accelerate and focus electrons in a free electron laser. Such magnetic fields are generated by large mass, current driven systems typically comprised of a solenoid, an external power source and a means for cooling the coils of the solenoid. One significant disadvantage of such conventional systems, however, is that they are large, expensive and energy-intensive.
The foregoing disadvantages are apparent when conventional electromagnetic apparatus are used to operate a free electron laser. A free electron laser typically has a passageway, which is often an excavated cylindrical space, through which an electron beam can pass. This passageway can be comprised of hard ferromagnetic material, i.e. permanent magnets, or it can be an iron Chiron Wiggler, which is a well-known type of free-electron laser structure affording much shorter magnetic periods than usual in such devices. The term "Chiron" is an abbreviation for Circular Hollow Iron, the material used to fabricate Chiron-type magnetic devices. References on Chiron Wigglers include R. H. Jackson et al., "The Coaxial Hybrid Iron Wiggler," Nuclear Instrumentation Methods, Physics Research, Vol. A (1994) and Leupold, Statutory Invention Registration No. H1615, entitled, "Magnetic Fields For Chiron Wigglers," issued on Dec. 3, 1996. In a Chiron Wiggler, an externally mounted solenoid can create an axial magnetic field along the length of the passageway. This axial field then saturates the passageway's material, i.e. iron, to induce a transverse magnetic field alternating in direction with displacement along the length of the passageway. Such a transverse field would accelerate and cause the electrons to oscillate circumferentially as they pass longitudinally along the passageway.
Wigglers accelerate electrons, so that they radiate energy, the frequency of which depends on the strength and periodicity of the magnetic field. However, the radiation produced by an electron beam oscillating in an alternating field is of a wave length commensurate with the period of the magnetic field, so that for short wavelengths it becomes difficult to space magnets sufficiently thin to operate and attain sufficient field strength for accelerating the electrons. This "tyranny of size" may be somewhat overcome by using relativistic electrons which "see" a magnet structure that shrinks with increasing electron energy. While such an arrangement works, it still requires massive equipment generating the high voltage needed to impart sufficient energy to the electrons to make them relativistic, which frustrates the goals of compactness and lightness. Until now, a permanent magnet structure addressing the long-felt problems of unattainably thin magnets of sufficient strength has been needed to simultaneously generate sufficient field strength and sufficiently short magnetic periods.
Many devices utilizing magnetic fields also need to provide a helically oriented transverse magnetic field. See, for example, Leupold, "Augmentation of Field Uniformity and Strength in Spherical and Cylindrical Magnetic Field Sources," 70 Journal of Applied Physics No. 2, p. 6621 (1991). Examples of such permanent magnetic devices include the structures found in Leupold, et. al., U.S. Pat. No. 4,764,743, entitled, "Permanent Magnet Structures For The Production of Transverse Helical Fields," issued on Aug. 16, 1988, Leupold, U.S. Pat. No. 4,994,778, entitled, "Adjustable Twister," issued on Feb. 19, 1991 and Leupold, U.S. Pat. No. 5,099,217, entitled, "Constant Gap Cladded Twister," issued on Mar. 24, 1992. Each of these references are incorporated herein by reference.
While these prior art structures all produce useful magnetic fields within a central working space, they are all relatively complex permanent magnet structures with numerous different magnets, making them disadvantageous in terms of complexity, weight and cost. Until now, the numerous advantages of the Chiron magnetic structures in the terms of electron acceleration and the desirable transverse helical magnetic fields of the magnetic twister have never been effectively combined to address the problems of higher field amplitudes and high power circularly polarized radiation, without suffering from the disadvantages of numerous complex magnetic pieces, high cost and excessive weight.
The present invention overcomes these disadvantages and offers numerous other advantages by conventional electromagnetic systems with a compact, light-weight and inexpensive Chiron Twister structure that provides the desired magnetic field and eliminates dependence on relatively expensive permanent magnets that are hard to fabricate and assemble. While the Chiron Wiggler's internal iron elements are saturated within the passageway to form a periodic pole distribution that creates a transverse sinusoidal magnetic field, the Chiron Twister forms a transverse helical field. The present invention addresses the problems of attaining higher field amplitudes in short periods, known as FEL's, for use in high power radiation sources by providing a source of high power circularly polarized radiation in a magnet structure comprising a Chiron Twister able to simultaneously generate both sufficient field strength and sufficiently short periods.
The present invention provides several embodiments of a simple, cost-effective and advantageous Chiron Twister structure providing the much-needed transverse helical magnetic fields that other magnet structures cannot provide, without any of the drawbacks, limitations and shortcomings of prior art magnetic structures in terms of complexity, cost and weight.